Plasma torches are frequently used for welding, hard facing, and cutting metals. These plasma torches have a torch head, to which a torch handle and a set of terminal components such as an electrode, a tip, and a shield cup are attached. The torch head has embedded therein various components for transferring fluids and electrical currents, which include a high frequency (HF) lead and a power lead/gas conduit. The power lead/gas conduit is separated from the HF lead by an insulating member. The torch head is fabricated from an electrically insulating and heat resistant material which is either a phenolic molding material or an elastomer such as a rubber compound. The torch head is typically formed by molding under pressure with the other components embedded therein.
For torch heads formed of heat resistant phenolic molding material, the life of the torch head is typically limited by fracturing of the torch head which is caused by abrasion of the surface in combination with thermal cycling of the torch head during use. To reduce such problems, multi-part torch heads have been fabricated having a core surrounded by a shell. The core segment is fabricated from a thermally stable epoxy or phenolic molding material which has a portion of the transfer components embedded therein. The core segment in turn is housed in a two-part shell cover, fabricated from a heat resistant phenolic molding material, which is fastened around the core segment. The use of a multiple part assembly allows safety switches to readily be incorporated into the torch head. The fabrication of the torch head from a multiple part assembly also reduces the mean thickness of the torch head elements and enhances the life of the torch head by reducing the thermal stresses. While the life of the multi-part torch head is greater than the life of the single part torch head, the resulting torch heads still suffer limited useful lives due to fracturing.
A discussion of the fabrication of torch heads is found in U.S. Pat. No. 5,519,185, which suggests that the use of elastomers for the housing is preferred to extend the usable life of the torch head. The '185 patent notes that the fabrication of housings from silicone rubber, which is one of the preferred elastomers for heat resistance, is particularly difficult since the flow characteristics of the silicone rubber are poor. The '185 patent teaches that torch heads formed from silicone rubber are usually formed by wrapping the components to be embedded in the housing with strips of silicone rubber and then pressing the resulting assembly in a mold to obtain the desired shape, with the excess material being squeezed out of the mold leaving a flash which is trimmed away.
It is noted in the '185 patent that these elastomer materials are subject to deterioration over time due to sloughing, and that this problem can be reduced by employing a three-layer structure having a first layer of a heat resistant elastomer, a second layer of a fibrous heat and abrasion resistant reinforcing material, and a third layer of a heat resistant elastomer. While the resulting structure provides a reduction in sloughing compared to single-layer elastomer torch heads, the improved life is obtained at the expense of complicating an already complex fabrication technique.
Thus, there is a need for a plasma torch head which is readily fabricated and which has a high resistance to damage from cyclic thermal stresses, mechanical wear, sloughing, and fracturing.